Curiosity about the cat has finally paid off with a scientific explanation for felines' enigmatic indifference to sweets.
Researchers, pet owners and cat chow manufacturers have long recognized that cats, in stark contrast to their canine counterparts, show no particular attraction to sugar. Having sampled two dishes of water, one spiked with sugar and the other not, a cat is as likely to lap from one as the other.
Until now, scientists have not known whether cats simply lack the lingual apparatus to detect sugar; or have functional sugar detectors on their tongues but faulty wiring from their taste buds to the brain; or - as some might presume - are simply too snooty to admit to such a common craving.
Now researchers studying the DNA of house cats, tigers and cheetahs have settled the question: Cats both large and small harbor a genetic mutation that renders the sugar detectors on their taste buds inoperative.
"We have found a simple but elegant explanation for their behavior," said Joseph G. Brand of the Monell Chemical Senses Center in Philadelphia, who with co-worker Xia Li led the study, published Sunday in the journal PLoS Genetics.
The work fills a gap in scientists' understanding of the evolution of cats, whose indifference to sugar - a rare trait among mammals - complements their complete dietary dependence on meat. In humans and other animals that depend on starches and ripe fruits for a sizable part of their nutrition, the ability to detect sugars is crucial.
Brand, who has two cats, said he could not say for sure how a cat's overall nature might be affected by never experiencing sweetness. But it may not be a coincidence, he said half-jokingly, that the dominant behavioral characteristics of a cat are that "it sleeps a lot and it's cranky."
In the new study, researchers collected DNA from six house cats, all of them pets of scientists at Monell. They analyzed the sequence of genetic "letters" in the two genes encoding the sweetness receptor's two proteins.
They found one of those genes, called Tas1r2, is missing a stretch of 247 letters - a deletion that prevents the gene from making a proper protein. With only one of the two crucial proteins, the cats have no way to taste sweetness.
The researchers found the same deletion in tiger and cheetah DNA, suggesting the mutation occurred in a common ancestor early in feline evolution.